1. Field of the Invention
The present invention relates to a sealing structure for sealing a space between a piston and a cylinder that are in sliding contact with each other and, more particularly, relates to a sealing structure in the form of a piston ring for an internal combustion engine wherein an outer ring made of a resinous material is pressed toward a sliding face by urging means via an inner ring. The present invention also relates to a method of mounting a piston ring structural body for use in the sealing structure.
2. Description of the Related Art
A sealing structure is widely employed to seal a space between a piston and a cylinder that are in sliding contact with each other.
For example, an internal combustion engine has a so-called piston ring disposed within a piston ring groove. Conventionally, in many cases, this piston ring is made of a metal. However, a resinous piston ring has also been developed.
For example, Japanese Patent Application Laid-Open No. HEI 9-280373 discloses usage of a resinous piston ring. In this publication, there is disclosed a piston ring structural body wherein an outer ring that is continuously formed of polytetrafluoroethylene (PTFE) as a fluororesin without being provided with an abutment is pressed radially outwards by a ring tensile force applying member composed of a metal coil expander via an inner ring that is also continuously formed of PTFE without being provided with an abutment.
In the piston ring structural body disclosed in the aforementioned publication, the ring is divided into the inner and outer rings. While the inner ring ensures sealability around a ring groove, the outer ring ensures sealability between a sliding portion thereof and a cylinder wall.
It is to be noted herein that the piston ring structural body disclosed in the aforementioned publication uses PTFE for both the inner and outer rings. However, PTFE has a low threshold value of PV (a product of a real pressure P and sliding speed V) and cannot provide the sliding portion that slides on the cylinder wall with sufficient durability. Furthermore, PTFE has a large coefficient of thermal expansion. Thus, when the temperature rises, the pressing force applied to the cylinder wall due to an elastic force of the coil expander becomes larger and increases a frictional loss of the engine, which raises a serious problem in relation to the outer ring.
Furthermore, when the temperature rises during operation of the engine, the coil expander cuts into the inner ring and causes permanent distortion. This results in a problem, for example, of deterioration in the sealability around the ring groove.